Production of beams of excited energetic neutral particles

ABSTRACT

Excited energetic neutral particle beams are produced, in accordance with one method, by directing a low energy ion beam through a gaseous medium under thick target conditions. In accordance with a second method, the low energy ions, i.e., below 10 KeV, are directed through a first selected gaseous medium at which resonance or near resonance charge exchange neutralization occurs and the neutral particles are then passed through a second gaseous medium in which the particles are excited to high nquantum levels. In either case the excited state neutral particle can then be more easily ionized by the Lorentz force when they are introduced into a magnetic containment field to form a plasma therein. The second gaseous medium generally comprises a different gas than the first and is especially selected to produce the maximum portion of excited states.

United States Patent Futch, Jr. et al.

[is] 3,657,542 [451 Apr. 18, 1972 [54] PRODUCTION OF BEAMS OF EXCITEDENERGETIC NEUTRAL PARTICLES [72] Inventors: Archer I-l. Futch, Jr.,Livermore, Calif.;

Robert H. McFarland, Rolla, Mo.

[73] Assignee: The United States of America as represented by the UnitedStates Atomic Energy Commission [22] Filed: May 4,1970

[2i] Appl. No.: 34,282

Primary Examiner-William F Lindquist Attorney-Roland A. AndersonABSTRACT Excited energetic neutral particle beams are produced, inaccordance with one method, by directing a low energy ion beam through agaseous medium under thick target conditions. in accordance with asecond method, the low energy ions, i.e., below 10 KeV, are directedthrough a first selected gaseous medium at which resonance or nearresonance charge exchange neutralization occurs and the neutralparticles are then passed through a second gaseous medium in which theparticles are excited to high n-quantum levels. in either case theexcited state neutral particle can then be more easily ionized by theLorentz force when they are introduced into a magnetic containment fieldto form a plasma therein. The second gaseous medium generally comprisesa different gas than the first and is especially selected to produce themaximum portion of excited states.

4 Claims, 4 Drawing Figures PATENTEUAPR 18 I972 SHEET 2 BF 2 INVENTORS.Robert H. MEFarland Archer H. Fulch, Jr.

ATTORNEY.

PRODUCTION OF BEAMS OF EXCITED ENERGETIC NEUTRAL PARTICLES BACKGROUND OFTHE INVENTION The invention described herein was conceived or made underContract W-7405-Eng-48 with the United States Atomic Energy Commission.

Energetic neutral atomic or molecular particles are employed in avariety of instruments or accelerators and especially in the productionof high temperature gases or plasmas such as those employed incontrolled fusion or thermonuclear devices and reactors. 1n the latterinstance, in accord with conventional practice, an energetic neutralparticle beam may be produced by directing a beam of relatively highenergy ions through a region containing a gaseous or vaporous targetmedium wherein the ions are neutralized by undergoing a charge exchangereaction with neutral atoms or molecules. A fraction of the emergingneutral particles are in highly excited quantum levels. The energeticneutral particles may then be directed into a magnetic containment fieldof appropriate configuration such as a magnetic mirror type field usedin a controlled fusion reactor. These neutral particles can penetrateexternal portions of the magnetic field to enter the magneticcontainment zone where the Lorentz field, V X B, equivalent to aspecific electric field E, is sufficient to ionize the highly excitedlevels. The charged particles formed by ionization or dissociation aretrapped in the containment zone to form a plasma. In this manner thedensity of the plasma may be increased to a desired operating level. Ifthe excited quantum state is too high, these particles may bedissociated and lost in external portions of the field and if too lowthe particles will pass through the system before ionization ordissociation occurs. Other dissociation mechanisms such as collisions ofunexcited energetic neutral particles with residual gas atoms andmolecules or with cold plasma are generally of subsidiary importance atleast in the initial phases of plasma density buildup. This procedurehas been widely used for injecting and trapping light nuclide ions,e.g., hydrogen isotopes in controlled fusion devices.

A typical controlled fusion device utilizing such a procedure is theALICE" device described, inter alia, in Section 9.179, ControlledThermonuclear Reactions, Glasstone and Lovberg, Van Nostrand Co., Inc.1960 pages 55-59; Plasma Physics and Thermonuclear Research, Series X1,Progress in Nuclear Energy Pergammon Press, 1963. The magnetic fieldconfiguration utilized in the foregoing devices is a steady magneticmirror field, i.e., an axial magnetic field having regions of increasedintensity at each end. Such a magnetic mirror field is a potential wellin which charged particles are contained. A modification thereof inwhich longitudinal conductors, i.e., Ioffe bars, are placed radiallyabout said magnetic field region produces a minimum B magnetic fieldwhich is stable with respect to hydromagnetic instabilities. Similarminimum B magnetic field configurations are produced by the so-calledtennis ball or baseball seam magnetic field coils, c.f., A Tennis BallSeam Coil for Plasma Physics Research, Proceedings of the SecondInternational Conference on Magnet Technology", Oxford, England, July-13, 1967. A baseball seam magnet for producing variable magnet fieldsis disclosed in the copending application of Carl D. Henning and AnthonyK. Chargin, Ser. No. 753,189, filed Aug. 16, 1968 issued as U.S. Pat.No. 3,491,318 on June 20, 1970. Devices utilizing conventionalconductors as well as super conductor magnetic field coils have beendesigned for producing the containment fields.

The interaction of a beam of energetic ions with atoms or molecules of avapor or gaseous medium involves three fundamental process, i.e.,production of neutral particles by charge exchange, scattering whichresults in losses from the beam, and excitation, or de-excitation ofenergetic neutral particles involving electrons in quantum states abovethe ground level, generally termed an excited state.

Heretofore, perhaps based on prior experience with high energy ions,i.e., above 10 KeV and particularly, at 20 KeV and above, conditionsproviding resonance or near resonance charge exchange of ions into theground level have been avoided. It has generally been considered thatrelatively low yields of excited neutral particles would be obtainedusing a target medium providing resonance or near resonance chargeexchange into the ground level.

Accordingly, in the prior art, the neutralizing medium has been selectedsuch that charge exchange of ions into the high excited levels was aresonance or near resonance process. As the target thickness of such aneutralizer is increased, the fraction of the ion beam which isneutralized increases, however multiple collisions were observed todecrease the fraction of neutral particles which are in highly excitedlevels. Therefore, a compromise target thickness was chosen to obtain anoptimum number of excited states with respect to the incident ion beam.

The charge exchange reaction yields en rgetic neutral particles whichare populated in their higher excited quantum states with a principalquantum number dependence roportional to n'. Furthermore, for a specificelectric field or an equivalent Lorentz field VX B it has been shownthat the fraction of neutral particles which are field ionized isproportional to E.

From the n dependence of the excited state population it has been shownthat for thin targets the ratio of the ions produced by the electricfield to the total neutrals is proportional to the square root of theionizing field: That is Neutrals field ionized by E and the ionizingfield, E, is proportional to n where a; is the probability of a chargeexchange interation leaving the neutral atom in the nth excited state.It has been found experimentally for thick targets that R(n) is alsoproportional to n. Therefore, one may define a as the correspondingproportionality constant for thick targets.

Target thicknesses may be expressed in terms of the gas pressure inmicrons (Hg) multiplied by the distance in centimeters traversed by theion beam, i.e., in the dimension of micron-centimeters. Targetthicknesses necessary to obtain equilibrium or thick target conditionsare dependent on the energy of the initial ions.

SUMMARY OF THE INVENTION The present invention relates generally to theproduction of beams of energetic neutral particles and, moreparticularly, to methods and apparatus for more efiiciently producingbeams of low energy neutral particles in excited levels.

It has now been discovered that the excited state populations of neutralatoms beams produced by charge exchange in a gas or vapor are enhancedat low energies (below about 10 Kev) by passage of the neutrals througha thick target. In providing resonance charge exchange conditions, it isan essential requirement that the atomic or molecular particles of thetarget medium possesses one or more quantum states which is closelyequivalent or similar to those of the energetic neutral atom or moleculeformed by charge exchange. This requirement is most easily satisfied byselecting a medium comprising similar atoms or corresponding molecularspecies. For example, with 1-1, D or T ions. hydrogen gas may be used. Asecond requirement necessary to enhance the excited state populations isthat the excitation rate predominates over the rate of loss ofexcitation for the selected charge exchange medium. This latterconditiifin does not appear to have been found possible to satisfyheretofore using higher energy ions. However, it has now been found thatthe condition can be satisfied with low energy ions so as to makepossible the more efficient production of beams of excited low energyneutral particles using certain procedure set forth hereinafter.Moreover, it has been found that the enhanced excitation effect persiststo thick target conditions so that a high proportion of excited neutralparticles can be produced.

Based on the foregoing discovery in accordance with one embodiment ofthe invention, a beam of low energy ions, i.c., below about Kev,particularly ions of the hydrogen isotopes protium (P), deuterium (D),and tritium (T), are directed through a region containing a selectedgaseous or vaporous resonance charge exchange medium under so-calledthick target" conditions to produce a beam of excited low energy neutralparticles. In a second embodiment of the invention, low energy ions arepassed through a first region containing a selected first gaseousneutralizing medium under resonance conditions and then through a secondregion containing a different gaseous medium having characteristicswhich more effectively enhance the populations of the excited levels inthe beam by collisional excitation. Thence forth, the excited neutralbeam may be directed into a magnetic field containment zone, e.g., of acontrolled fusion reactor, wherein the particles dissociate in theLorentz force field therein, to form a plasma.

Apparatus for producing the beam generally includes an evacuated housingcontaining an ion source. In the case of the single-medium proceduredescribed above, a collimat'ed ion beam is directed from the ion sourcethrough a neutralizer cell which interposes a selected gaseous medium.e.g., hydrogen operated under selected thick target conditions tointeract with the ions and yield a beam of excited neutral particles. Inthe second case the collimated ion beam is directed through aneutralizer which interposes a selected gaseous medium under conditionsmost effective for conversion of the ions into neutral particles. Theneutral particle beam therefrom is then directed through an exciterwhich interposes a second medium operating under conditions whichenhance the populations of excited levels in the beam by collisionalexcitation with the medium.

Accordingly it is an object of the invention to provide apparatus andmethods for producing low energy excited neutral particles for use inestablishing plasmas in magnetically defined containment zones whichobjective may be accomplished either by passing a charged particle beamthrough a vaporous medium which accomplishes both neutralization andexcitation or by passing a beam through a gaseous medium first to effectneutralization and thenceforth through a second vaporous medium toenhance the population of excited levels in the neutral beam.

Other objects and advantageous features of the invention will beapparent in the following description taken in conjunction with theaccompanying drawings, of which:

FIG. I is a graphical illustration of the variation in excited energeticneutral particle production from hydrogen and deuterium ions of varyingenergy interacting with a resonant 10 utilizing a first gaseous targetmedium for neutralizing energetic charged particles and a second gaseousmedium for exciting the energetic neutrals by collisional processes.

Procedures for measuring excited state populations by the use ofelectric field ionization has been reported by many is authors, see forexample Enhancement of the Excited-State Population in a Hydrogen AtomBeam" A. H. Futch et al., Nuclear Fusion, 3, I24 (1963) and referencescited therein. For the present purpose results of certain determinationsare expressed in terms of the thin target excitation coeficient, a,,,

the excitation coefficient for thick targets, a, and R(1l) or R( l4)which are, a.,, or a, divided by (1]) and (I4) respectively. Results ofmeasurements of, a obtained in the course of directing H at 25, 10 and 5KeV energies and D at a 10 KeV energy through an H, charge exchangemedium, are graphically illustrated in FIG. 1 of the drawing. Withrespect to atomic collisions, D at 10 KeV energy is equivalent to H at 5KeV. The Kev values are typical of those known heretofore and show thehighly eflective production of excited neutral particles using thintargets wherein single collisions predominate as well as the dramaticdecrease in production as the target thickness is increased. Thisdecrease is generally attributed to depopulation of the excited energystates by the multiple collisions which occur in the thick targets. At10 KeV it may be observed that excited state neutral production remainssubstantially constant over the range of thin to thick targets. However,it may be noted that with 5 KeV H ions as well as for the equivalent l0Kev D ions, excited state neutral production is at a relatively lowlevel for thin targets,

but increases quite steadily as target thickness is increased up to anoptimal thickness.

The results of a further study of the proportion of excited l0 KeVneutral deuterium atoms interacting with H, gas under varied targetthickness conditions are illustrated in FIG. 2 of the drawing. It willbe observed, that with incident I0 KeV,

D", (5 KeV, H*), 300 micron-cm, approximates an optimal target thicknessand that the rate of change, e.g., between about 80 micron-cm and 500micron-cm, is relatively low. The foregoing values may therefore betaken to indicate the limits of the practical operating range.Characteristic operating curves such as the foregoing may be used toselect the conditions to be used in the apparatus described hereinafter.For the optimal value of target thickness, i.c., 200 micron-cm, onemight use a gas cell of about 100 cm length operating at 2 microns H, orD, pressure. Various other combinations of cell length and gas pressuregiving comparable target thicknesses may be selected. Results ofmeasurement made with a variety of other media and at various ionenergies for H are given in Table l.

TABLE L'COMPILA'IION OF MEASUREMENTS OF 'IIIIN AND THICK TARGETEXCITATION COEFFICIENTS [Numbers in parentheses are target thicknessesfor maximal excited neutral production In micron-cm.)

Energy,kev. 7 i 4 W s s 7.5 10 12 5 I5 20 2a 30 Molecule a a a n as a doa an a a a a a n a a, a a a Mg. .4. .i, 1.3. l 2.6. 'll .72 l 1.3 L5 ll.,m; (201)).12 .15 .ll .28 N (l.l (27ll).l!l.. (5 1-3 .23. .46 U .51 H 60Fm 1.1) (IISU).35 .8 (HIIL35 .41 .38 .36 (Bil).3l (7[)).33 (5").35 g{40).33

Target thickness corresponding to thick targets are listed enclosed inparenthesis above the a values in the thick target coefficient, a,column therein. Thin target, a,,, coefficients are also listed.

Comparison of additional data similar to FIG. 1 and 2 for the productionof field ionized neutrals (for the same ion current) at S KeV equivalentenergy shows H to be the best medium, of those listed, for producingexcited state low-energy neutral hydrogen particles. Relative to Hneutralizing media composed of N NH C F and H appear effective by thefactors 0.32, 0.18, 0.17, and 0. l4, respectively, at op timalneutralizer thickness as employed in the single medium embodimentdescribed above. These ratios reflect charge exchange and scatteringcross sections in addition to the a value. These results at low ionenergies are categorically different from those expected from priorexperience and represent a marked increase as a class thereover althoughthe individual materials vary within the class.

In the second embodiment of the invention, a hydrogen, magnesium, orsimilar target having a thickness in the range of about 50-150 micron-cmwith an optimum value of about l00 micron-cm may be used in the firstneutralizer unit. In the second neutralizer unit per fluorodimethylcyclohexane (C F H O, H NH or nitrogen gas (N,) with a targetthickness in the range of about 50 to 300 micron-cm may be used.

Apparatus for producing a beam of low energy excited neutral particlesutilizing a single thick target medium in accordance with the firstembodiment of the invention, is illustrated in FIG. 3. Such apparatusmay comprise an elongated hermetically sealed enclosure 11, having anion source 12 mounted to direct a beam 13, of low energy ions, i.e., H*,D or T in the range of about 1 KeV to l0 KeV and particularly in therange of about 2 KeV to about 8 KeV longitudinally along the enclosure.The ion source 12 may be of any conventional design which produces awell collimated beam of low energy ions, e.g., the well-knownDuo-Plasmatron type described in the Plasma Physics and ThermonuclearResearch" reference cited above. A gas neutralizer unit I4 is positionedsuch that channel 16 is in coaxial alignment with the ion beam 13. A gasmay be admitted to the central tubular member 17 defining a portion ofchannel 16 in unity from a conduit 18 leading to a controlled flow gassupply (not shown) exterior to housing 11. In a neutralizer of this typea gas or vapor is chosen such that charge exchange of ions into highlyexcited levels is a resonance or near resonance process. The targetthickness is then determined by the requirement that the excited statepopulation be a maximum. With KeV, H, D or T, particles the optimaltarget thickness for H, or D in the range of about 80 micron-cm to about500 micron-cm.

In the neutralizer channel 16 the low energy ions undergo multiplecollisions with the neutral atoms or molecules of the target mediumfirst to undergo a charge exchange reaction producing neutral particleswhich then are excited to higher quantum states which emerge in anessentially collimated beam 19 of excited neutral particles. Using Dions and H gas as an example, the overall reaction may be illustrated asfollows:

The excited neutral particle beam 19 is directed through a beam tube 21into the vacuum vessel 22 of a controlled fusion reactor or other hightemperature plasma device of the con ventional character describedabove, to be ionized and trapped to form a plasma in the magnetic fieldcontainment zone thereof as in conventional practice.

The beam tube 21 may include a valve 22 for closing off the vacuumvessel during servicing and maintenance operations. One or more vacuumpumps 23 may be coupled thereto to remove any neutral gas or othercontaminant escaping from enclosure 11. The beam tube 21 may alsoinclude a baffled inner wall surface which, with coolantcoils, (notshown) serve to condense and trap materials such a target media escapingfrom enclosure 11, thereby preventing the introduction of undesiredcontaminants into vessel 22, in a manner well known in the art. A beamtube constructed in this manner may comprise a differential pumpingarrangement of the character disclosed on page 57, of said PlasmaPhysics and Thermonuclear Research" reference cited above. Enclosure 11may be provided with one or more vacuum pumps, for example, a diffusionpump 24 and/or a high capacity gettering pump 26 or other equivalenthigh capacity vacuum pumps (not shown). In usual practice, enclosure 11and beam tube 21 are evacuated to below about 10* mm Hg. Vessel 22 ispreferably evacuated to below about 10" to l0 mm Hg. in accord withconventional controlled fusion reactor practice utilizing additionalvacuum pumps (not shown). In the event a condensible vaporous medium isemployed in such a unit 14, a temperature regulated heat transfer mediummay be circulated through tubing coils 28 wound about tubular member 17to maintain the vapor at an appropriate temperature level. Likewise, acryogenic coolant, e.g., liquid nitrogen or other appropriate coolantmay be circulated through coils 29, 29' disposed about the ends of unit14 if needed to cool baffles 3| defining the ends of channel 16 tocondense and minimize escape of medium from unit 14. The rate of flow ofsaid medium from conduit 18 may be regulated to establish the desiredpressure in channel 16. The pressure of the medium in the channelmeasured in microns (Hg) multiplied by the length of channel 16 yieldsthe target thickness of the medium therein.

For operating in accord with the second embodiment of the invention, theapparatus of FIG. 3 is modified to include an additional neutralizerunit which is interposed within enclosure ll between the source 12 andneutralizer unit 14. In this instance the gas or vapor in the additionalunit is chosen so that charge exchange into the ground level, i.e.,charge exchange neutralization, occurs as a resonance process resultingin a maximum preparation of neutral particles in the beam which emergestherefrom. The unit 14, otherwise similar to unit 14 now, however, issupplied through conduit 18 with a gaseous or vaporous medium of such anature especially selected to provide optimal excitation and minimumloss of particles from beam 41 passing therethrough. The excited stateenergetic neutral particle beam 42 emergent from unit 14' is thendirected through beam tube 21 as with neutral particle beam 19 describedabove. Since neutralization and excitation can each be optimized, withthis dual tandem neutralizer arrangement, more efficient production ofan excited state particle beam can be realized.

The additional unit may be similar in construction to units 14 or 14when a gaseous medium such as hydrogen is employed therein. However,highly efficient neutralization may also be obtained using a vaporousmaterial such as magnesium vapor which requires a neutralizer unit 42capable of operation at an elevated temperature such as that indicatedin FIG. 4, in order to obtain an adequate vapor pressure. Moreparticularly the enclosure 11 is elongated to accommodate unit 42 whichcomprises a closed vessel portion 43 suspended from a cover plate 44secured in sealed relation to the periphery of an opening in the upperwall 46 of enclosure 11' and defines a chamber 47 therein. Electricalheating elements 48 and a heat shield 49 are arranged in proximity tothe sidewalls and bottom of vessel portion 43 to provide heat forvaporizing a material, such as magnesium chips 51, heated to atemperature in the range of about 300 to 500 C., to provide theneutralizer medium target thickness in chamber 47. Openings 52, 52 areprovided in the sidewalls of vessel portion 43 aligned with openings 53,53' in shield 49 to allow passage of ion beam 13 through chamber 47 tointeract with said magnesium vapor and emerge as neutral beam 41 whichthen passes through unit 14' as described above. It will be appreciatedthat a gaseous medium introduced in any suitable manner may also beemployed in such a unit 42.

The reaction in chamber 47 may be represented by the equation:

D +Mg=D+Mg* with hydrogen in such a chamber or other additionalneutralizer, mentioned above, the reaction is The low energy neutralparticles, e.g., D", on undergoing multiple collisions during passagethrough the thick target medium, in unit [4' are raised to an excitedenergy level, i.e., the electrons are raised to higher quantum levelsshort of ionization. The interaction may be represented by the followingequation:

In summary in operating the foregoing apparatus, a collimated beam 13 oflow energy ions, for example, in the range of about 800 eV to about KeV,is directed through the neutralizer unit 42 which contains a vaporous orgaseous neutralizing medium. A medium may be used therein which providesresonant or near resonance charge exchange interaction so that maximalneutralization is achieved without a compromise which might be requiredif optimal excitation was concurrently desired. These conditions aresuch that a major proportion of the ions undergo a single collisioneffective to produce neutralization by Charge exchange while a minorproportion undergo multiple collisions which would cause excessivescattering from the beam so that a high proportion of the injected ionsemerge as neutral particles in the collimated beam 41. Now when thecollimated beam 41 of energetic neutral particles passes through unit14', the particles undergo multiple collisions with the target vapormedium therein under conditions such that a maximal proportion areexcited so that the electrons thereof are raised to higher, more easilyionizable or dissociable quantum levels. The excited neutral particlebeam 42, emerging from unit 14 may accordingly be directed through thebeam tube 21 into the magnetic containment zone (not shown) in reactorvessel 22. The excited neutral particles are ionized initially, atleast, by Lorentz fields as mentioned above, to initiate the formationof a plasma, e.g., a thermonuclear plasma containing D, T*, D*--Tmixture, or the like. However, as the plasma density builds up theenergy of the ions delivered by source 12 may be raised and conditionsin the neutralizer and/or exciter modified to produce higher energyexcited neutral particles, e.g., to I00 KeV, which can now, in thepresence of the initiator plasma, be efiectively ionized and trapped tofurther build up the plasma density as well as elevate the plasmatemperature to approach or attain thermonuclear reaction conditions inthe reactor. Alternatively, separate high energy neutral particlesources of conventional type may be used in combination with the lowenergy excited neutral particle sources provided as described above.

While there has been described in the foregoing what may be consideredto be preferred embodiments of the invention, modification may be madetherein within the spirit and scope of the invention and it is intendedto cover all such as fall within the scope of the appended claims.

What we claim is: 1. In a process for producing a beam of excited stateenergetic neutral particles, the steps comprising:

generating a collimated beam of ions comprising a material selected fromthe group consisting of H", D and T, said ions having an energy of belowabout 10 KeV; and directing said beam of ions through a regioncontaining a gaseous medium selected from the group consisting of H,, NC F and H 0, said gaseous medium presenting a tar get thickness in therange of about micron-cm to about 500 micron-cm wherein said ionsundergo multiple collisions with the particles of said gaseous mediumand are converted into neutral particles in an excited quantum state andwherefrom said beam emerges from said region as said beam of excitedstate neutral particles. 2. A process as defined in claim 1 wherein saidions of said collimated beam have an energy in the range of about 0.8KeV to about 8 KeV.

. la a process for producing a beam of excited state energetic neutralparticles, the steps comprising:

generating a collimated beam of ions comprising a material selected fromthe group consisting of H, D and T, said ions having an energy of belowabout 10 Kev;

directing said collimated beam of ions through a first region containinga gaseous medium operated under resonance charge exchange conditionswherein the ions undergo charge exchange neutralization to emerge as aneutral particle beam; and

then directing said neutral particle beam through a region containing asecond gaseous medium selected from the group consisting of H 0, H Nl-lN and C F said second gaseous medium presenting a target thickness inthe range of about 30 micron-cm to about lOO microncm.

4. A process as defined in claim 3 wherein said gaseous medium operatedunder resonance charge exchange conditions is a material selected fromthe group consisting of hydrogen and magnesium under conditionspresenting a target thickness in the range of about 50 micron-cm toabout micron-cm.

1. In a process for producing a beam of excited state energetic neutralparticles, the steps comprising: generating a collimated beam of ionscomprising a material selected from the group consisting of H , D and T, said ions having an energy of below about 10 KeV; and directing saidbeam of ions through a region containing a gaseous medium selected fromthe group consisting of H2, N2, C8F16 and H2O, said gaseous mediumpresenting a target thickness in the range of about 80 micron-cm toabout 500 micron-cm wherein said ions undergo multiple collisions withthe particles of said gaseous medium and are converted into neutralparticles in an excited quantum state and wherefrom said beam emergesfrom said region as said beam of excited state neutral particles.
 2. Aprocess as defined in claim 1 wherein said ions of said collimated beamhave an energy in the range of about 0.8 KeV to about 8 KeV.
 3. In aprocess for producing a beam of excited state energetic neutralparticles, the steps comprising: generating a collimated beam of ionscomprising a material selected from the group consisting of H , D and T, said ions having an energy of below about 10 KeV; directing saidcollimated beam of ions through a first region containing a gaseousmedium operated under resonance charge exchange conditions wherein theions undergo charge exchange neutralization to emerge as a neutralparticle beam; and then directing said neutral particle beam through aregion containing a second gaseous medium selected from the groupconsisting of H2O, H2, NH3, N2 and C8F16, said second gaseous mediumpresenting a target thickness in the range of about 30 micron-cm toabout 100 micron-cm.
 4. A process as defined in claim 3 wherein saidgaseous medium operated under resonance charge exchange conditions is amaterial selected from the group consisting of hydrogen and magnesiumunder conditions presenting a target thickness in the range of about 50micron-cm to about 150 micron-cm.